System Boilers and Stored Hot Water
A system boiler heats a cylinder rather than heating water on demand.
A system boiler heats a cylinder rather than heating water on demand. It suits homes where more than one outlet gets used at the same time, and it keeps mains-pressure showers strong because the cylinder is fed from the mains.
The pairing that matters
The boiler and the cylinder have to be sized together. An oversized boiler on a small cylinder short-cycles; a small boiler on a large cylinder means long reheats.
Recovery, not capacity
People fixate on litres. What decides whether you run out is the coil rating and how quickly the cylinder recovers after a bath.
Where the components live
Pump, diverter or zone valves and expansion vessel usually sit near the cylinder, which makes the airing cupboard the service point rather than the boiler itself.
Questions we get asked
Is a system boiler the same as a regular boiler?
No. A system boiler has the pump and expansion built in and runs sealed; a regular boiler needs external feed and expansion tanks.
Can I keep my existing cylinder?
Sometimes, if it is a modern indirect unvented cylinder in good condition. An old vented copper cylinder usually has to go when you seal the system.
Does an unvented cylinder need annual attention?
Yes. The safety devices and the vessel charge should be checked each year by someone qualified for unvented work.